This invention relates to solid state lasers and specifically such lasers that have a region in their active layer provided with a lateral spatial thickness variation (LSTV) in their active layers. Such spatial variations are also referred to as laterally tapered active regions in diode lasers.
One class of lasers incorporating such an active region are known as channelled substrate nonplanar lasers (CSNP), the term "nonplanar" denoting that the active layer possesses a nonplanar active region, which region is formed during fabrication of the laser. The nonplanar region is conventionally formed via liquid phase epitaxy (LPE), by growth of laser semiconductor layers over a channelled substrate. An example of such a laser is disclosed in U.S. Pat. No. 3,978,428. Such nonplanar laser structures may be fabricated in other ways, as will be discussed later.
Various studies have been made on the geometries and properties of LSTV type injection lasers.
Examples of such analyses are disclosed in "Channeled Substrate Buried Heterostructure GaAs-(GaAl)As Injection Lasers", P. A. Kirkby and G. H. B. Thompson Journal of Applied Physics, Vol. 47, No. 10, page 4578 et seq. (October, 1976) and "Channeled-Substrate Narrow-Stripe GaAs/GaAlAs Injection Lasers With Extremely Low Threshold Currents", P. A. Kirkby Electronic Letters, Vol. 15 No. 25 pages 824-826 (December, 1979).
The strong interest in these types of lasers is attributed to their good operating characteristics, such as, low threshold current, linear light versus current characteristic, good temperature performance, and relatively symmetrical, anastigmatic output beams.
A principal shortcoming of these devices, however, has been that LSTV laser will begin to lase simultaneously in the fundamental (TE.sub.00) and first order (TE.sub.01) and/or other higher order modes not far above their operating threshold. This condition is not accompanied by a distinctive kink in the optical output power versus current characteristic but nevertheless it is undesirable in laser applications that require highly focused output beams over a wide range of output power.
Model consideration of LSTV lasers is therefore highly desirable in order to achieve laser designs that operate at single fundamental transverse mode at elevated power output levels without the higher order transverse modes while simultaneously maintaining or improving other favorable geometric and operating characteristics. Such analysis includes studies and experiments involving property variations with parametric changes, such as, dependencies on layer thicknesses, layer compositions, layer thickness variations, laser length, current confinement with widths, spreading resistance, and carrier diffusion constants.
We have approached this problem from the standpoint of viewing these parametric changes from studies and experiments relating to the TE.sub.00 mode power level at which the TE.sub.01 mode attains threshold, which threshold is termed P.sub.1 *.